Widespread functional specialization of Plasmodium falciparum erythrocyte membrane protein 1 family members to bind CD36 analysed across a parasite genome

Plasmodium falciparum var gene expression is modified by host immunity

Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) is a potentially important family of immune targets, which play a central role in the host-parasite interaction by binding to various host molecules. They are encoded by a diverse family of genes called var, of which there are approximately 60 copies in each parasite genome. In sub-Saharan Africa, although P. falciparum infection occurs throughout life, severe malarial disease tends to occur only in childhood. This could potentially be explained if (i) PfEMP1 variants differ in their capacity to support pathogenesis of severe malaria and (ii) this capacity is linked to the likelihood of each molecule being recognized and cleared by naturally acquired antibodies. Here, in a study of 217 Kenyan children with malaria, we show that expression of a group of var genes "cys2," containing a distinct pattern of cysteine residues, is associated with low host immunity. Expression of cys2 genes was associated with parasites from young children, those with severe malaria, and those with a poorly developed antibody response to parasite-infected erythrocyte surface antigens. Cys-2 var genes form a minor component of all genomic var repertoires analyzed to date. Therefore, the results are compatible with the hypothesis that the genomic var gene repertoire is organized such that PfEMP1 molecules that confer the most virulence to the parasite tend also to be those that are most susceptible to the development of host immunity. This may help the parasite to adapt effectively to the development of host antibodies through modification of the host-parasite relationship.

A model of the complex between the PfEMP1 malaria protein and the human ICAM-1 receptor

Malaria is caused by protozoan parasites of the genus Plasmodium. Four species of Plasmodium can infect humans: P. falciparum, P. malariae, P. vivax, and P. ovale. P. falciparum is the only able to cytoadhere to the surface of postcapillary endothelial cells. A key role in cytoadherence is played by the interaction between the PfEMP1 P. falciparum protein and the human intracellular adhesion molecule (ICAM-1) although very little is known about the molecular details of this complex. Here we propose a model for this interaction on the basis of a homology model of the functional domain of PfEMP1 and of the ICAM-1 three dimensional structures. Our model is consistent with the results of many experimental observations, provides a rational explanation for the different binding abilities of different strains of P. falciparum and explains the reduced binding affinity of the A4 strain of P. falciparum for the ICAM-1(Kilifi) polymorphism. On the basis of our model, we can also explain why the murine ICAM-1, although sharing 70% sequence similarity with its human homologue, does not bind PfEMP1, and why the binding of fibrinogen and PfEMP1 to ICAM-1 is mutually exclusive. The model of the complex proposed here can serve as a useful tool for the design and interpretation of biochemical and immunological experimental results.

Adhesion of Plasmodium falciparum-infected erythrocytes to human cells: molecular mechanisms and therapeutic implications

Severe malaria has a high mortality rate (15–20%) despite treatment with effective antimalarial drugs. Adjunctive therapies for severe malaria that target the underlying disease process are therefore urgently required. Adhesion of erythrocytes infected with Plasmodium falciparum to human cells has a key role in the pathogenesis of life-threatening malaria and could be targeted with antiadhesion therapy. Parasite adhesion interactions include binding to endothelial cells (cytoadherence), rosetting with uninfected erythrocytes and platelet-mediated clumping of infected erythrocytes. Recent research has started to define the molecular mechanisms of parasite adhesion, and antiadhesion therapies are being explored. However, many fundamental questions regarding the role of parasite adhesion in severe malaria remain unanswered. There is strong evidence that rosetting contributes to severe malaria in sub-Saharan Africa; however, the identity of other parasite adhesion phenotypes that are implicated in disease pathogenesis remains unclear. In addition, the possibility of geographic variation in adhesion phenotypes causing severe malaria, linked to differences in malaria transmission levels and host immunity, has been neglected. Further research is needed to realise the untapped potential of antiadhesion adjunctive therapies, which could revolutionise the treatment of severe malaria and reduce the high mortality rate of the disease.

A var gene promoter implicated in severe malaria nucleates silencing and is regulated by 3' untranslated region and intronic cis-elements

Questions surround the mechanism of mutually exclusive expression by which Plasmodium falciparum mediates activation and silencing of var genes. These encode PfEMP1 proteins, which function as cytoadherent and immunomodulatory molecules at the surface of parasitised erythrocytes. Current evidence suggests that promoter silencing by var introns might play a key role in var gene regulation. To evaluate the impact of cis-acting regulatory regions on var silencing, we generated P. falciparum lines in which luciferase was placed under the control of an UpsA var promoter. By utilising the Bxb1 integrase system, these reporter cassettes were targeted to a genomic region that was not in apposition to var subtelomeric domains. This eliminated possible effects from surrounding telomeric elements and removed the variability inherent in episomal systems. Studies with highly synchronised parasites revealed that the UpsA element possessed minimal activity in comparison with a heterologous (hrp3) promoter. This may result from the integrated UpsA promoter being largely silenced by the neighbouring cg6 promoter. Our analyses also revealed that the DownsA 3' untranslated region further decreased the luciferase activity from both cassettes, whereas the var A intron repressed the UpsA promoter specifically. By applying multivariate analysis over the entire cell cycle, we confirmed the significance of these cis-elements and found the parasite stage to be the major factor regulating UpsA-promoter activity. Additionally, we observed that the UpsA promoter was capable of nucleating reversible silencing that spread to a downstream promoter. We believe these studies are the first to analyse promoter activity of Group A var genes, which have been implicated in severe malaria, and support the model that var introns can further suppress var expression. These data also suggest an important suppressive role for the DownsA terminator. Our findings imply the existence of multiple levels of var gene regulation in addition to intrinsic promoter-dependent silencing.

High throughput functional assays of the variant antigen PfEMP1 reveal a single domain in the 3D7 Plasmodium falciparum genome that binds ICAM1 with high affinity and is targeted by naturally acquired neutralizing antibodies

Plasmodium falciparum–infected erythrocytes bind endothelial receptors to sequester in vascular beds, and binding to ICAM1 has been implicated in cerebral malaria. Binding to ICAM1 may be mediated by the variant surface antigen family PfEMP1: for example, 6 of 21 DBLβC2 domains from the IT4 strain PfEMP1 repertoire were shown to bind ICAM1, and the PfEMP1 containing these 6 domains are all classified as Group B or C type. In this study, we surveyed binding of ICAM1 to 16 DBLβC2 domains of the 3D7 strain PfEMP1 repertoire, using a high throughput Bioplex assay format. Only one DBL2βC2 domain from the Group A PfEMP1 PF11_0521 showed strong specific binding. Among these 16 domains, DBL2βC2_{PF11_0521} best preserved the residues previously identified as conserved in ICAM1-binding versus non-binding domains. Our analyses further highlighted the potential role of conserved residues within predominantly non-conserved flexible loops in adhesion, and, therefore, as targets for intervention. Our studies also suggest that the structural/functional DBLβC2 domain involved in ICAM1 binding includes about 80 amino acid residues upstream of the previously suggested DBLβC2 domain. DBL2βC2_{PF11_0521} binding to ICAM1 was inhibited by immune sera from east Africa but not by control US sera. Neutralizing antibodies were uncommon in children but common in immune adults from east Africa. Inhibition of binding was much more efficient than reversal of binding, indicating a strong interaction between DBL2βC2_{PF11_0521} and ICAM1. Our high throughput approach will significantly accelerate studies of PfEMP1 binding domains and protective antibody responses.

Plasmodium falciparum exports the protein PfEMP1 to the surface of parasitized erythrocytes for roles in immunoevasion and adhesion. The size and structural complexity of this diverse protein family have limited earlier studies of PfEMP1 biology to low throughput and semi-quantitative approaches. We developed a high throughput quantitative assay of PfEMP1 adhesion and used it to analyze structural features of domains that bind the putative cerebral receptor ICAM1, as well as to survey the acquisition of functional antibodies in malaria-exposed children and adults. In studies of the PfEMP1 repertoire of clone 3D7 parasites, a single specific domain bound ICAM1 strongly. PfEMP1 domains that bind ICAM1 strongly have conserved features, including conserved amino acids within otherwise highly variant flexible loops of the protein. While neutralizing antibodies against the PfEMP1–ICAM1 interaction were uncommon in Tanzanian children, such antibodies were common in east African adults, possibly explaining why immune adults rarely carry ICAM1-binding parasites. This high throughput platform will significantly accelerate studies of PfEMP1 binding domains and the corresponding antibody responses involved in protective immunity.

CD36 selection of 3D7 Plasmodium falciparum associated with severe childhood malaria results in reduced VAR4 expression

Background

A subset of the Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1_SM) is involved in the cytoadherence of P. falciparum-infected red blood cells (iRBC) contributing to the pathogenesis of severe disease among young children in malaria endemic areas. The PfEMP1_SM are encoded by group A var genes that are composed of a more constrained range of amino acid sequences than groups B and C var genes encoding PfEMP1_UM associated with uncomplicated malaria. Also, unlike var genes from groups B and C, those from group A do not have sequences consistent with CD36 binding – a major cytoadhesion phenotype of P. falciparum isolates.

Methods

A 3D7 PfEMP1_SM sub-line (3D7_SM) expressing VAR4 (PFD1235w/MAL8P1.207) was selected for binding to CD36. The protein expression of this parasite line was monitored by surface staining of iRBC using VAR4-specific antibodies. The serological phenotype of the 3D7_SM parasites was determined by flow cytometry using malaria semi-immune and immune plasma and transcription of the 59 var genes in 3D7 were analysed by real-time quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) using var-specific primers.

Results

A selection-induced increased adhesion of 3D7_SM iRBC to CD36 resulted in a reduced var4 transcription and VAR4 surface expression.

Conclusion

VAR4 is not involved in CD36 adhesion. The current findings are consistent with the notion that CD36 adhesion is not associated with particular virulent parasite phenotypes, such as those believed to be exhibited by VAR4 expressing parasites.

An in vivo and in vitro model of Plasmodium falciparum rosetting and autoagglutination mediated by varO, a group A var gene encoding a frequent serotype

In the Saimiri sciureus monkey, erythrocytes infected with the varO antigenic variant of the Plasmodium falciparum Palo Alto 89F5 clone bind uninfected red blood cells (rosetting), form autoagglutinates, and have a high multiplication rate, three phenotypic characteristics that are associated with severe malaria in human patients. We report here that varO parasites express a var gene having the characteristics of group A var genes, and we show that the varO Duffy binding-like 1alpha(1) (DBL1alpha(1)) domain is implicated in the rosetting of both S. sciureus and human erythrocytes. The soluble varO N-terminal sequence (NTS)-DBL1alpha(1) recombinant domain, produced in a baculovirus-insect cell system, induced high titers of antibodies that reacted with varO-infected red blood cells and disrupted varO rosettes. varO parasites were culture adapted in vitro using human erythrocytes. They formed rosettes and autoagglutinates, and they had the same surface serotype and expressed the same varO gene as the monkey-propagated parasites. To develop an in vitro model with highly homogeneous varO parasites, rosette purification was combined with positive selection by panning with a varO NTS-DBL1alpha(1)-specific mouse monoclonal antibody. The single-variant, clonal parasites were used to analyze seroprevalence for varO at the village level in a setting where malaria is holoendemic (Dielmo, Senegal). We found 93.6% (95% confidence interval, 89.7 to 96.4%) seroprevalence for varO surface-reacting antibodies and 86.7% (95% confidence interval, 82.8 to 91.6%) seroprevalence for the recombinant NTS-DBL1alpha(1) domain, and virtually all permanent residents had seroconverted by the age of 5 years. These data imply that the varO model is a relevant in vivo and in vitro model for rosetting and autoagglutination that can be used for rational development of vaccine candidates and therapeutic strategies aimed at preventing malaria pathology.

The cysteine-rich interdomain region from the highly variable plasmodium falciparum erythrocyte membrane protein-1 exhibits a conserved structure

Plasmodium falciparum malaria parasites, living in red blood cells, express proteins of the erythrocyte membrane protein-1 (PfEMP1) family on the red blood cell surface. The binding of PfEMP1 molecules to human cell surface receptors mediates the adherence of infected red blood cells to human tissues. The sequences of the 60 PfEMP1 genes in each parasite genome vary greatly from parasite to parasite, yet the variant PfEMP1 proteins maintain receptor binding. Almost all parasites isolated directly from patients bind the human CD36 receptor. Of the several kinds of highly polymorphic cysteine-rich interdomain region (CIDR) domains classified by sequence, only the CIDR1α domains bind CD36. Here we describe the CD36-binding portion of a CIDR1α domain, MC179, as a bundle of three α-helices that are connected by a loop and three additional helices. The MC179 structure, containing seven conserved cysteines and 10 conserved hydrophobic residues, predicts similar structures for the hundreds of CIDR sequences from the many genome sequences now known. Comparison of MC179 with the CIDR domains in the genome of the P. falciparum 3D7 strain provides insights into CIDR domain structure. The CIDR1α three-helix bundle exhibits less than 20% sequence identity with the three-helix bundles of Duffy-binding like (DBL) domains, but the two kinds of bundles are almost identical. Despite the enormous diversity of PfEMP1 sequences, the CIDR1α and DBL protein structures, taken together, predict that a PfEMP1 molecule is a polymer of three-helix bundles elaborated by a variety of connecting helices and loops. From the structures also comes the insight that DBL1α domains are approximately 100 residues larger and that CIDR1α domains are approximately 100 residues smaller than sequence alignments predict. This new understanding of PfEMP1 structure will allow the use of better-defined PfEMP1 domains for functional studies, for the design of candidate vaccines, and for understanding the molecular basis of cytoadherence.

Malaria parasites express proteins of the erythrocyte membrane protein-1 family (PfEMP1) on the surfaces of the human red blood cells that they infect. These large proteins vary in sequence extensively, yet bind to host receptors to allow infected cells to adhere to host tissues. PfEMP1 proteins help parasites evade the immune system, as the 60 PfEMP1 genes are expressed one at a time. Sequence comparisons predict that PfEMP1 molecules are modular, made up of Duffy binding-like (DBL) and cysteine-rich interdomain region (CIDR) domains. Many CIDR domains bind to the human receptor CD36. We have analyzed the structure of the CD36-binding portion, known as MC179, of a CIDR domain. The MC179 protein is composed of a bundle of three helices connected by a loop and three additional helices. Based on the structure and sequence similarities, MC179 is a good model for the hundreds of known CIDR sequences. In addition, the MC179 three-helix bundle is remarkably similar to subdomain 3 of the known DBL structures. MC179 provides insight into the relatedness of both kinds of PfEMP1 domains and predicts that the large PfEMP1 molecules are polymers of three-helix bundles and their connecting polypeptides.

Plasmodium falciparum antigenic variation. Mapping mosaic var gene sequences onto a network of shared, highly polymorphic sequence blocks

Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) is a potentially important family of immune targets, encoded by an extremely diverse gene family called var. Understanding of the genetic organization of var genes is hampered by sequence mosaicism that results from a long history of non-homologous recombination. Here we have used software designed to analyse social networks to visualize the relationships between large collections of short var sequences tags sampled from clinical parasite isolates. In this approach, two sequences are connected if they share one or more highly polymorphic sequence blocks. The results show that the majority of analysed sequences including several var-like sequences from the chimpanzee parasite Plasmodium reichenowi can be either directly or indirectly linked together in a single unbroken network. However, the network is highly structured and contains putative subgroups of recombining sequences. The major subgroup contains the previously described group A var genes, previously proposed to be genetically distinct. Another subgroup contains sequences found to be associated with rosetting, a parasite virulence phenotype. The mosaic structure of the sequences and their division into subgroups may reflect the conflicting problems of maximizing antigenic diversity and minimizing epitope sharing between variants while maintaining their host cell binding functions.

Frequent recombination events generate diversity within the multi-copy variant antigen gene families of Plasmodium falciparum

The human malaria parasite Plasmodium falciparum utilises a mechanism of antigenic variation to avoid the antibody response of its human host and thereby generates a long-term, persistent infection. This process predominantly results from systematic changes in expression of the primary erythrocyte surface antigen, a parasite-produced protein called PfEMP1 that is encoded by a repertoire of over 60 var genes in the P. falciparum genome. var genes exhibit extensive sequence diversity, both within a single parasite's genome as well as between different parasite isolates, and thus provide a large repertoire of antigenic determinants to be alternately displayed over the course of an infection. Whilst significant work has recently been published documenting the extreme level of diversity displayed by var genes found in natural parasite populations, little work has been done regarding the mechanisms that lead to sequence diversification and heterogeneity within var genes. In the course of producing transgenic lines from the original NF54 parasite isolate, we cloned and characterised a parasite line, termed E5, which is closely related to but distinct from 3D7, the parasite used for the P. falciparum genome nucleotide sequencing project. Analysis of the E5 var gene repertoire, as well as that of the surrounding rif and stevor multi-copy gene families, identified examples of frequent recombination events within these gene families, including an example of a duplicative transposition which indicates that recombination events play a significant role in the generation of diversity within the antigen encoding genes of P. falciparum.

The C-terminal segment of the cysteine-rich interdomain of Plasmodium falciparum erythrocyte membrane protein 1 determines CD36 binding and elicits antibodies that inhibit adhesion of parasite-infected erythrocytes

Attachment of erythrocytes infected by Plasmodium falciparum to receptors of the microvasculature is a major contributor to the pathology and morbidity associated with malaria. Adhesion is mediated by the P. falciparum erythrocyte membrane protein 1 (PfEMP-1), which is expressed at the surface of infected erythrocytes and is linked to both antigenic variation and cytoadherence. PfEMP-1 contains multiple adhesive modules, including the Duffy binding-like domain and the cysteine-rich interdomain region (CIDR). The interaction between CIDRalpha and CD36 promotes stable adherence of parasitized erythrocytes to endothelial cells. Here we show that a segment within the C-terminal region of CIDRalpha determines CD36 binding specificity. Antibodies raised against this segment can specifically block the adhesion to CD36 of erythrocytes infected with various parasite strains. Thus, small regions of PfEMP-1 that determine binding specificity could form suitable components of an antisequestration malaria vaccine effective against different parasite strains.

Sub-grouping and sub-functionalization of the RIFIN multi-copy protein family

Background

Parasitic protozoans possess many multicopy gene families which have central roles in parasite survival and virulence. The number and variability of members of these gene families often make it difficult to predict possible functions of the encoded proteins. The families of extra-cellular proteins that are exposed to a host immune response have been driven via immune selection to become antigenically variant, and thereby avoid immune recognition while maintaining protein function to establish a chronic infection.

Results

We have combined phylogenetic and function shift analyses to study the evolution of the RIFIN proteins, which are antigenically variant and are encoded by the largest multicopy gene family in Plasmodium falciparum. We show that this family can be subdivided into two major groups that we named A- and B-RIFIN proteins. This suggested sub-grouping is supported by a recently published study that showed that, despite the presence of the Plasmodium export (PEXEL) motif in all RIFIN variants, proteins from each group have different cellular localizations during the intraerythrocytic life cycle of the parasite. In the present study we show that function shift analysis, a novel technique to predict functional divergence between sub-groups of a protein family, indicates that RIFINs have undergone neo- or sub-functionalization.

Conclusion

These results question the general trend of clustering large antigenically variant protein groups into homogenous families. Assigning functions to protein families requires their subdivision into meaningful groups such as we have shown for the RIFIN protein family. Using phylogenetic and function shift analysis methods, we identify new directions for the investigation of this broad and complex group of proteins.

Var transcription profiling of Plasmodium falciparum 3D7: assignment of cytoadherent phenotypes to dominant transcripts

BACKGROUND

Cytoadherence of Plasmodium falciparum-infected red blood cells is mediated by var gene-encoded P. falciparum erythrocyte membrane protein-1 and host receptor preference depends in most cases on which of the 50-60 var genes per genome is expressed. Enrichment of phenotypically homogenous parasites by panning on receptor expressing cells is fundamental for the identification of the corresponding var transcript.

METHODS

P. falciparum 3D7 parasites were panned on several transfected CHO-cell lines and their var transcripts analysed by i) reverse transcription/PCR/cloning/sequencing using a universal DBLalpha specific oligonucleotide pair and ii) by reverse transcription followed by quantitative PCR using 57 different oligonucleotide pairs.

RESULTS

Each cytoadherence selected parasite line also adhered to untransfected CHO-745 cells and upregulation of the var gene PFD995/PFD1000c was consistently associated with cytoadherence to all but one CHO cell line. In addition, parasites panned on different CHO cell lines revealed candidate var genes which reproducibly associated to the respective cytoadherent phenotype. The transcription profile obtained by RT-PCR/cloning/sequencing differed significantly from that of RT-quantitative PCR.

CONCLUSION

Transfected CHO cell lines are of limited use for the creation of monophenotypic cytoadherent parasite lines. Nevertheless, 3D7 parasites can be reproducibly selected for the transcription of different determined var genes without genetic manipulation. Most importantly, var transcription analysis by RT-PCR/cloning/sequencing may lead to erroneous interpretation of var transcription profiles.

Plasmodium falciparum erythrocyte membrane protein-1 specifically suppresses early production of host interferon-gamma

Plasmodium falciparum erythrocyte membrane protein-1 (PfEMP-1) is a variable antigen expressed by P. falciparum, the malarial parasite. PfEMP-1, present on the surface of infected host erythrocytes, mediates erythrocyte binding to vascular endothelium, enabling the parasite to avoid splenic clearance. In addition, PfEMP-1 is proposed to regulate host immune responses via interactions with the CD36 receptor on antigen-presenting cells. We investigated the immunoregulatory function of PfEMP-1 by comparing host cell responses to erythrocytes infected with either wild-type parasites or transgenic parasites lacking PfEMP-1. We showed that PfEMP-1 suppresses the production of the cytokine interferon-gamma by human peripheral blood mononuclear cells early after exposure to P. falciparum. Suppression of this rapid proinflammatory response was CD36 independent and specific to interferon-gamma production by gammadelta-T, NK, and alphabeta-T cells. These data demonstrate a parasite strategy for downregulating the proinflammatory interferon-gamma response and further establish transgenic parasites lacking PfEMP-1 as powerful tools for elucidating PfEMP-1 functions.

Mapping a common interaction site used by Plasmodium falciparum Duffy binding-like domains to bind diverse host receptors

The Duffy binding-like (DBL) domain is a key adhesive module in Plasmodium falciparum, present in both erythrocyte invasion ligands (EBLs) and the large and diverse P. falciparum erythrocyte membrane protein 1 (PfEMP1) family of cytoadherence receptors. DBL domains bind a variety of different host receptors, including intercellular adhesion molecule 1 (ICAM-1), a receptor interaction that may have a role in infected erythrocyte binding to cerebral blood vessels and cerebral malaria. In this study, we expressed the nearly full complement of DBLbeta-C2 domains from the IT4/25/5 (IT4) parasite isolate and showed that ICAM-1-binding domains (DBLbeta-C2(ICAM-1)) were confined to group B and group C PfEMP1 proteins and were not present in group A, suggesting that ICAM-1 selection pressure differs between PfEMP1 groups. To further dissect the molecular determinants of binding, we modelled a DBLbeta-C2(ICAM-1) domain on a solved DBL structure and created alanine substitution mutants in two DBLbeta-C2(ICAM-1) domains. This analysis indicates that the DBLbeta-C2::ICAM-1 interaction maps to the equivalent glycan binding region of EBLs, and suggests a general model for how DBL domains evolve under dual selection for host receptor binding and immune evasion.

Characterization of the antibody response against Plasmodium falciparum erythrocyte membrane protein 1 in human volunteers

The immune response against the Plasmodium falciparum variant surface antigen P. falciparum erythrocyte membrane protein 1 (PfEMP1) is a key component of clinical immunity against falciparum malaria. In this study, we used sera from human volunteers who had been infected with the P. falciparum 3D7 strain to investigate the development, specificity, and dynamics of anti-PfEMP1 antibodies measured against six different strain 3D7 Duffy binding-like domain 1alpha (DBL1alpha) fusion proteins. We observed that a parasitemia of 20 to 200 infected erythrocytes per mul was required to trigger an antibody response to DBL1alpha and that antibodies against one DBL1alpha variant cross-react with other DBL1alpha variants. Both serum and purified immunoglobulin Gs (IgGs) were able to agglutinate infected erythrocytes, and purified anti-DBL1alpha IgGs bound to the live infected red blood cell surface in a punctate surface pattern, confirming that the IgGs recognize native PfEMP1. Analysis of sera from tourists naturally infected with P. falciparum suggests that the anti-PfEMP1 antibodies often persisted for more than 100 days after a single infection. These results help to further our understanding of the development of acquired immunity to P. falciparum infections.

Disruption of var2csa gene impairs placental malaria associated adhesion phenotype

Infection with Plasmodium falciparum during pregnancy is one of the major causes of malaria related morbidity and mortality in newborn and mothers. The complications of pregnancy-associated malaria result mainly from massive adhesion of Plasmodium falciparum-infected erythrocytes (IE) to chondroitin sulfate A (CSA) present in the placental intervillous blood spaces. Var2CSA, a member of the P. falciparum erythrocyte membrane protein 1 (PfEMP1) family is the predominant parasite ligand mediating CSA binding. However, experimental evidence suggests that other host receptors, such as hyaluronic acid (HA) and the neonatal Fc receptor, may also support placental binding. Here we used parasites in which var2csa was genetically disrupted to evaluate the contribution of these receptors to placental sequestration and to identify additional adhesion receptors that may be involved in pregnancy-associated malaria. By comparison to the wild-type parasites, the FCR3Δvar2csa mutants could not be selected for HA adhesion, indicating that var2csa is not only essential for IE cytoadhesion to the placental receptor CSA, but also to HA. However, further studies using different pure sources of HA revealed that the previously observed binding results from CSA contamination in the bovine vitreous humor HA preparation. To identify CSA-independent placental interactions, FCR3Δvar2csa mutant parasites were selected for adhesion to the human placental trophoblastic BeWo cell line. BeWo selected parasites revealed a multi-phenotypic adhesion population expressing multiple var genes. However, these parasites did not cytoadhere specifically to the syncytiotrophoblast lining of placental cryosections and were not recognized by sera from malaria-exposed women in a parity dependent manner, indicating that the surface molecules present on the surface of the BeWo selected population are not specifically expressed during the course of pregnancy-associated malaria. Taken together, these results demonstrate that the placental malaria associated phenotype can not be restored in FCR3Δvar2csa mutant parasites and highlight the key role of var2CSA in pregnancy malaria pathogenesis and for vaccine development.

The role of leukocytes bearing Natural Killer Complex receptors and Killer Immunoglobulin-like Receptors in the immunology of malaria

The biology of Natural Killer (NK) cells and other NK Receptor (NKR)(+) leukocytes has largely been elucidated in viral or cancer systems, and involvement in other diseases or infectious states is less clearly defined. Recently, however, clear evidence has emerged for a role in malaria. NK cells and NKR(+) leukocytes significantly control susceptibility and resistance to both malaria infection and severe disease syndromes in murine models, in dependence upon receptors encoded within the Natural Killer Complex (NKC). Plasmodium falciparum can rapidly activate human NKR(+) gammadelta T cells and NK cells in vitro, and these responses are controlled partly by NKR loci encoded within the human syntenic NKC and Killer Immunoglobulin-like Receptor (KIR) genomic regions. Neither erythrocytes nor malaria parasites express HLA or MHC Class I-like homologues, or obvious stress-type ligands, suggesting the possibility of novel NKR recognition mechanisms. Parasite-derived ligands such as P. falciparum Erythrocyte Membrane Protein-1 (PfEMP-1) and glycosylphosphatidylinositol (GPI) regulate some of these diverse responses. Population-based immunogenetic analyses should allow the identification of NKC and KIR loci controlling innate and adaptive immune responses to malaria and associated with altered risk of infection and disease.

Cytoadhesion of Plasmodium falciparum-infected erythrocytes and the infected placenta: a two-way pathway

Malaria is undoubtedly the world's most devastating parasitic disease, affecting 300 to 500 million people every year. Some cases of Plasmodium falciparum infection progress to the deadly forms of the disease responsible for 1 to 3 million deaths annually. P. falciparum-infected erythrocytes adhere to host receptors in the deep microvasculature of several organs. The cytoadhesion of infected erythrocytes to placental syncytiotrophoblast receptors leads to pregnancy-associated malaria (PAM). This specific maternal-fetal syndrome causes maternal anemia, low birth weight and the death of 62,000 to 363,000 infants per year in sub-Saharan Africa, and thus has a poor outcome for both mother and fetus. However, PAM and non-PAM parasites have been shown to differ antigenically and genetically. After multiple pregnancies, women from different geographical areas develop adhesion-blocking antibodies that protect against placental parasitemia and clinical symptoms of PAM. The recent description of a new parasite ligand encoded by the var2CSA gene as the only gene up-regulated in PAM parasites renders the development of an anti-PAM vaccine more feasible. The search for a vaccine to prevent P. falciparum sequestration in the placenta by eliciting adhesion-blocking antibodies and a cellular immune response, and the development of new methods for evaluating such antibodies should be key priorities in mother-child health programs in areas of endemic malaria. This review summarizes the main molecular, immunological and physiopathological aspects of PAM, including findings related to new targets in the P. falciparum var gene family. Finally, we focus on a new methodology for mimicking cytoadhesion under blood flow conditions in human placental tissue.

Six genes are preferentially transcribed by the circulating and sequestered forms of Plasmodium falciparum parasites that infect pregnant women

In areas of stable malaria transmission, susceptibility to Plasmodium falciparum malaria increases during first pregnancy. Women become resistant to pregnancy malaria over successive pregnancies as they acquire antibodies against the parasite forms that sequester in the placenta, suggesting that a vaccine is feasible. Placental parasites are antigenically distinct and bind receptors, like chondroitin sulfate A (CSA), that are not commonly bound by other parasites. We used whole-genome-expression analysis to find transcripts that distinguish parasites of pregnant women from other parasites and employed a novel approach to define and adjust for cell cycle timing of parasites. Transcription of six genes was substantially higher in both placental parasites and peripheral parasites from pregnant women, and each gene encodes a protein with a putative export sequence and/or transmembrane domain. This cohort of genes includes var2csa, a member of the variant PfEMP1 gene family previously implicated in pregnancy malaria, as well as five conserved genes of unknown functions. Women in East Africa acquire antibodies over successive pregnancies against a protein encoded by one of these genes, PFD1140w, and this protein shows seroreactivity similar to that of VAR2CSA domains. These findings suggest that a suite of genes may be important for the genesis of the placental binding phenotype of P. falciparum and may provide novel targets for therapeutic intervention.

Differential var gene expression in the organs of patients dying of falciparum malaria

Sequestration of parasitized erythrocytes in the microcirculation of tissues is thought to be important in the pathogenesis of severe falciparum malaria. A major variant surface antigen, var/Plasmodium falciparum erythrocyte membrane protein 1, expressed on the surface of the infected erythrocyte, mediates cytoadherence to vascular endothelium. To address the question of tissue-specific accumulation of variant types, we used the unique resource generated by the clinicopathological study of fatal paediatric malaria in Blantyre, Malawi, to analyse var gene transcription in patients dying with falciparum malaria. Despite up to 102 different var genes being expressed by P. falciparum populations in a single host, only one to two of these genes were expressed at high levels in the brains and hearts of these patients. These major var types differed between organs. However, identical var types were expressed in the brains of multiple patients from a single malaria season. These results provide the first evidence of organ-specific accumulation of P. falciparum variant types and suggest that parasitized erythrocytes can exhibit preferential binding in the body, supporting the hypothesis of cytoadherence-linked pathogenesis.

Cytoadherence and genotype of Plasmodium falciparum strains from symptomatic children in Franceville, southeastern Gabon

BACKGROUND

Plasmodium falciparum causes severe clinical manifestations by sequestering parasitized red blood cells (PRBC) in the microvasculature of major organs such as the brain. This sequestration results from PRBC adherence to vascular endothelial cells via erythrocyte membrane protein 1, a variant parasite surface antigen.

OBJECTIVE

To determine whether P. falciparum multiple genotype infection (MGI) is associated with stronger PRBC cytoadherence and greater clinical severity.

METHODS

Nested polymerase chain reaction was used to genotype P. falciparum isolates from symptomatic children and to distinguish between single genotype infection (SGI) and MGI. PRBC cytoadhesion was studied with cultured human lung endothelial cells.

RESULTS

Analysis of two highly polymorphic regions of the merozoite surface antigen (MSP)-1 and MSP-2 genes and a dimorphic region of the erythrocyte binding antigen-175 gene showed that 21.4% and 78.6% of the 42 children had SGI and MGI, respectively. It also showed that 37 (89%) of the 42 PRBC samples expressed MSP-1 allelic family K1. Cytoadherence values ranged from 58 to 1811 PRBC/mm(2) of human lung endothelial cells monolayer in SGI and from 5 to 5744 PRBC/mm(2) in MGI. MGI was not associated with higher cytoadherence values or with more severe malaria.

CONCLUSIONS

These results suggested that infection of the same individual by multiple clones of P. falciparum does not significantly influence PRBC cytoadherence or disease severity and confirmed the predominance of the MSP-1 K1 genotype in southeastern Gabon.

Structural modifications to a high-activity binding peptide located within the PfEMP1 NTS domain induce protection against P. falciparum malaria in Aotus monkeys

Binding of P. falciparum-infected erythrocytes to vascular endothelium and to uninfected erythrocytes is mediated by the parasite-derived variant erythrocyte membrane protein PfEMP-1 and various receptors, both on the vascular endothelium and on the erythrocyte surface. Consecutive, non-overlapping peptides spanning the N-terminal segment (NTS) and Duffy-binding-like PfEMP1 sequence alpha-domain (DBLalpha) of this protein were tested in erythrocyte and C32 cell binding assays. Eight peptides specifically bound to C32 cells, and were named high-activity binding peptides (HABPs). No erythrocyte binding HABPs were found in this region. Strikingly, three HABPs [6504 ((1)MVELA KMGPK EAAGG DDIED(20)), 6505 ((21)ESAKH MFDRI GKDVY DKVKE(40)) and 6506 ((41)YRAKE RGKGL QGRLS EAKFEK(60))] are located within the NTS, for which no specific function has yet been described. HABP 6505 is neither immunogenic nor protection-inducing; therefore, based on our previous reports, critical amino acids (shown in bold) in HABP-C32 cell binding were identified and replaced to modify HABP immunogenicity and protectivity. Analogue peptide 12722 (ESAKH KFDRI GKDVY DMVKE) produced high antibody titres and completely protected three out of 12 vaccinated Aotus monkeys and 23410 (KHKFD FIGKI VYDMV KER) also produced high protection-inducing titres and completely protected one out of eight monkeys. (1)H NMR studies showed that all peptides were helical. Binding of these peptides to isolated HLADRbeta1 molecules did not reveal any preference, suggesting that they could bind to molecules not studied here.

Patterns of gene recombination shape var gene repertoires in Plasmodium falciparum: comparisons of geographically diverse isolates

Background

Var genes encode a family of virulence factors known as PfEMP1 (Plasmodium falciparum erythrocyte membrane protein 1) which are responsible for both antigenic variation and cytoadherence of infected erythrocytes. Although these molecules play a central role in malaria pathogenesis, the mechanisms generating variant antigen diversification are poorly understood. To investigate var gene evolution, we compared the variant antigen repertoires from three geographically diverse parasite isolates: the 3D7 genome reference isolate; the recently sequenced HB3 isolate; and the IT4/25/5 (IT4) parasite isolate which retains the capacity to cytoadhere in vitro and in vivo.

Results

These comparisons revealed that only two var genes (var1csa and var2csa) are conserved in all three isolates and one var gene (Type 3 var) has homologs in IT4 and 3D7. While the remaining 50 plus genes in each isolate are highly divergent most can be classified into the three previously defined major groups (A, B, and C) on the basis of 5' flanking sequence and chromosome location. Repertoire-wide sequence comparisons suggest that the conserved homologs are evolving separately from other var genes and that genes in group A have diverged from other groups.

Conclusion

These findings support the existence of a var gene recombination hierarchy that restricts recombination possibilities and has a central role in the functional and immunological adaptation of var genes.

Differential var gene transcription in Plasmodium falciparum isolates from patients with cerebral malaria compared to hyperparasitaemia

The Plasmodium falciparum variant erythrocyte surface antigens known as PfEMP1, encoded by the var gene family, are thought to play a crucial role in malaria pathogenesis because they mediate adhesion to host cells and immuno-modulation. Var genes have been divided into three major groups (A, B and C) and two intermediate groups (B/A and B/C) on the basis of their genomic location and upstream sequence. We analysed expressed sequence tags of the var gene DBLα domain to investigate var gene transcription in relation to disease severity in Malian children. We found that P. falciparum isolates from children with cerebral malaria (unrousable coma) predominantly transcribe var genes with DBLα1-like domains that are characteristic of Group A or B/A var genes. In contrast, isolates from children with equally high parasite burdens but no symptoms or signs of severe malaria (hyperparasitaemia patients) predominantly transcribe var genes with DBLα0-like domains that are characteristic of the B and C-related var gene groups. These results suggest that var genes with DBLα1-like domains (Group A or B/A) may be implicated in the pathogenesis of cerebral malaria, while var genes with DBLα0-like domains promote less virulent malaria infections.

Differential expression of var gene groups is associated with morbidity caused by Plasmodium falciparum infection in Tanzanian children

The var gene family of Plasmodium falciparum encodes the variant surface antigen Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1). PfEMP1 is considered an important pathogenicity factor in P. falciparum infection because it mediates cytoadherence to host cell endothelial receptors. var genes can be grouped into three major groups, A, B, and C, and the conserved var genes, var1-4, according to sequence similarities in coding and noncoding upstream regions. Using real-time quantitative PCR in a study conducted in Tanzania, the var transcript abundances of the different var gene groups were compared among patients with severe, uncomplicated, and asymptomatic malaria. Transcripts of var group A and B genes were more abundant in patients with severe malaria than in patients with uncomplicated malaria. In general, the transcript abundances of var group A and B genes were higher for children with clinical malaria than for children with asymptomatic infections. The var group C and var1-like transcript abundances were similar between the three sample groups. A transcript abundance pattern similar to that for var group A was observed for var2csa and var3-like genes. These results suggest that substantial and systematic differences in var gene expression exist between different clinical presentations.

A family affair: var genes, PfEMP1 binding, and malaria disease

An immunovariant adhesion protein family in Plasmodium falciparum named erythrocyte membrane protein 1 (PfEMP1), encoded by var genes, is responsible for both antigenic variation and cytoadhesion of infected erythrocytes at blood microvasculature sites throughout the body. Elucidation of the genome sequence of P. falciparum has revealed that var genes can be classified into different groups, each with distinct 5' flanking sequences, chromosomal locations and gene orientations. Recent binding and serological comparisons suggest that this genomic organization might cause var genes to diversify into separately recombining adhesion groups that have different roles in infection and disease. Detailed understanding of PfEMP1 expression and receptor binding mechanisms during infection and of the antigenic relatedness of disease variants might lead to new approaches in prevention of malaria disease.

Plasmodium falciparum variant surface antigen expression patterns during malaria

The variant surface antigens expressed on Plasmodium falciparum–infected erythrocytes are potentially important targets of immunity to malaria and are encoded, at least in part, by a family of var genes, about 60 of which are present within every parasite genome. Here we use semi-conserved regions within short var gene sequence “tags” to make direct comparisons of var gene expression in 12 clinical parasite isolates from Kenyan children. A total of 1,746 var clones were sequenced from genomic and cDNA and assigned to one of six sequence groups using specific sequence features. The results show the following. (1) The relative numbers of genomic clones falling in each of the sequence groups was similar between parasite isolates and corresponded well with the numbers of genes found in the genome of a single, fully sequenced parasite isolate. In contrast, the relative numbers of cDNA clones falling in each group varied considerably between isolates. (2) Expression of sequences belonging to a relatively conserved group was negatively associated with the repertoire of variant surface antigen antibodies carried by the infected child at the time of disease, whereas expression of sequences belonging to another group was associated with the parasite “rosetting” phenotype, a well established virulence determinant. Our results suggest that information on the state of the host–parasite relationship in vivo can be provided by measurements of the differential expression of different var groups, and need only be defined by short stretches of sequence data.

Hope that it will be possible to develop a malaria vaccine is supported by the fact that individuals who have grown up in malaria endemic regions learn to carry malarial infections without suffering disease. Surprisingly little is still known about how this immunity develops. Much current research focuses on how the host develops immune responses to parasite antigens that are exposed to the host immune system. A major family of such antigens are inserted into the surface of parasite-infected erythrocytes, where they undergo antigenic switching to evade a developing antibody response. These proteins are encoded by a family of approximately 60 var genes, variants of which are present in every parasite genome.

The extreme diversity of the var genes has prevented meaningful comparison of their expression in clinical isolates. However, the authors of this paper show that var genes can be placed in groups that have a similar representation in the genomes of all parasites that the authors collected from Kenyan children. Having demonstrated an underlying similarity at the genomic level, the authors show that the var expression patterns vary markedly between different patients. The expression levels of specific groups of var genes was associated with poorly developed antibody responses in the children and a well-established parasite virulence phenotype. The study provides tools for exploring how host and parasite adapt to one another as immunity develops.

Transcriptome analysis of antigenic variation in Plasmodium falciparum--var silencing is not dependent on antisense RNA

A microarray analysis of Plasmodium falciparum selected to express different var genes suggests that antisense transcripts are not responsible for the transcriptional silencing of non-expressed var genes.

Background

Plasmodium falciparum, the causative agent of the most severe form of malaria, undergoes antigenic variation through successive presentation of a family of antigens on the surface of parasitized erythrocytes. These antigens, known as Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) proteins, are subject to a mutually exclusive expression system, and are encoded by the multigene var family. The mechanism whereby inactive var genes are silenced is poorly understood. To investigate transcriptional features of this mechanism, we conducted a microarray analysis of parasites that were selected to express different var genes by adhesion to chondroitin sulfate A (CSA) or CD36.

Results

In addition to oligonucleotides for all predicted protein-coding genes, oligonucleotide probes specific to each known var gene of the FCR3 background were designed and added to the microarray, as well as tiled sense and antisense probes for a subset of var genes. In parasites selected for adhesion to CSA, one full-length var gene (var2csa) was strongly upregulated, as were sense RNA molecules emanating from the 3' end of a limited subset of other var genes. No global relationship between sense and antisense production of var genes was observed, but notably, some var genes had coincident high levels of both antisense and sense transcript.

Conclusion

Mutually exclusive expression of PfEMP1 proteins results from transcriptional silencing of non-expressed var genes. The distribution of steady-state sense and antisense RNA at var loci are not consistent with a silencing mechanism based on antisense silencing of inactive var genes. Silencing of var loci is also associated with altered regulation of genes distal to var loci.

Clinical and molecular aspects of severe malaria

The erythrocytic cycle of Plasmodium falciparum presents a particularity in relation to other Plasmodium species that infect man. Mature trophozoites and schizonts are sequestered from the peripheral circulation due to adhesion of infected erythrocytes to host endothelial cells. Modifications in the surface of infected erythrocytes, termed knobs, seem to facilitate adhesion to endothelium and other erythrocytes. Adhesion provides better maturation in the microaerophilic venous atmosphere and allows the parasite to escape clearance by the spleen which recognizes the erythrocytes loss of deformability. Adhesion to the endothelium, or cytoadherence, has an important role in the pathogenicity of the disease, causing occlusion of small vessels and contributing to failure of many organs. Cytoadherence can also describe adhesion of infected erythrocytes to uninfected erythrocytes, a phenomenon widely known as rosetting. Clinical aspects of severe malaria, as well as the host receptors and parasite ligands involved in cytoadherence and rosetting, are reviewed here. The erythrocyte membrane protein 1 of P. falciparum (PfEMP1) appears to be the principal adhesive ligand of infected erythrocytes and will be discussed in more detail. Understanding the role of host receptors and parasite ligands in the development of different clinical syndromes is urgently needed to identify vaccination targets in order to decrease the mortality rates of this disease.

Expression of Plasmodium falciparum erythrocyte membrane protein 1 in experimentally infected humans

Background

Parasites causing severe malaria in non-immune patients express a restricted subset of variant surface antigens (VSA), which are better recognized by immune sera than VSA expressed during non-severe disease in semi-immune individuals. The most prominent VSA are the var gene-encoded Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) family, which is expressed on the surface of infected erythrocytes where it mediates binding to endothelial receptors. Thus, severe malaria may be caused by parasites expressing PfEMP1 variants that afford parasites optimal sequestration in immunologically naïve individuals and high effective multiplication rates.

Methods

var gene transcription was analysed using real time PCR and PfEMP1 expression by western blots as well as immune plasma recognition of parasite cultures established from non-immune volunteers shortly after infection with NF54 sporozoites.

Results

In cultures representing the first generation of parasites after hepatic release, all var genes were transcribed, but GroupA var genes were transcribed at the lowest levels. In cultures established from second or third generation blood stage parasites of volunteers with high in vivo parasite multiplication rates, the var gene transcription pattern differed markedly from the transcription pattern of the cultures representing first generation parasites. This indicated that parasites expressing specific var genes, mainly belonging to group A and B, had expanded more effectively in vivo compared to parasites expressing other var genes. The differential expression of PfEMP1 was confirmed at the protein level by immunoblot analysis. In addition, serological typing showed that immune sera more often recognized second and third generation parasites than first generation parasites.

Conclusion

In conclusion, the results presented here support the hypothesis that parasites causing severe malaria express a subset of PfEMP1, which bestows high parasite growth rates in individuals with limited pre-existing immunity.

Functional interdependence of the DBLbeta domain and c2 region for binding of the Plasmodium falciparum variant antigen to ICAM-1

Cytoadherence of Plasmodium falciparum-infected erythrocytes is associated with severe malaria and is primarily mediated through binding of the variant surface antigen P. falciparum erythrocyte membrane protein 1 (PfEMP1) to specific host ligands. Infected erythrocyte binding to Intercellular Adhesion Molecule 1 (ICAM-1) has been implicated as having a role in cerebral malaria, a major cause of death from P. falciparum infection. We have examined ICAM-1-binding PfEMP1 proteins in the cytoadhesive P. falciparum strain IT4/25/5 in order to extend our understanding of binding. For A4tres, the ICAM-1 binding region was previously shown to reside within contiguous DBL2beta and c2 domains. We determined the gene sequence encoding IT-ICAM var, and showed that ICAM-1 binding in this protein also maps to DBL2betac2 domains that have 48% amino acid identity to A4tres. By truncation and chimera analysis, most of the DBL2beta and the first half of the c2 region were required for A4tres binding to ICAM-1, suggesting this tandem should be considered a structural-functional combination for ICAM-1 binding. Of interest, a chimera formed between two different ICAM-1 binding domains did not bind ICAM-1, suggesting a functional interdependence between DBL2beta and c2 from the same protein. As gene recombination and gene conversion are important mechanisms for generating diversity in the PfEMP1 protein family, this finding implies an extra level of constraint on the functional evolution of binding traits. Knowledge about the PfEMP1::ICAM-1 interaction may allow the development of interventions to prevent binding and disease.

Pregnancy-associated malaria and the prospects for syndrome-specific antimalaria vaccines

Aided by the Plasmodium falciparum genome project, recent discoveries regarding the molecular basis of malaria pathogenesis have led to a better understanding of the interactions between host and parasite. Although vaccines that prevent infection by malaria parasites remain only hopes for the future, there are now more immediate prospects for vaccines that protect against specific disease syndromes. Here, we discuss the latest advances in the development of a vaccine that specifically targets pregnancy-associated malaria (PAM).

Variant surface antigens, virulence genes and the pathogenesis of malaria

The first Molecular Approaches to Malaria meeting was held 2-5 February 2000 in Lorne, Australia. Following the meeting, Brian Cooke, Mats Wahlgren and Ross Coppel predicted that research into the molecular details of the mechanisms behind sequestration of parasitized erythrocytes would "become increasingly more complicated, with further interactions, receptors, ligands and functional domains". Furthermore, they cautioned that "the challenge will be not to lose ourselves in the molecular detail, but remain focused on the role of [the var genes and other multigene families] in pathogenesis of malaria". We contemplate on these statements, following the recent second Molecular Approaches to Malaria meeting, which was held at the same venue on 2-5 February 2004.

Malaria and the red blood cell membrane

Malaria is the most serious and widespread parasitic disease of humans and is arguably the commonest disease of red blood cells (RBCs). Malaria has exerted a powerful effect on human evolution and selection for resistance has led to the appearance and persistence of a number of inherited diseases. After parasite invasion, RBCs are progressively and dramatically modified. New structures appear inside the RBC and novel parasite proteins are exported to the erythrocyte cytoplasm and membrane skeleton. Radical biochemical, morphological, and rheological alterations manifest as increased membrane rigidity, reduced cell deformability, and greater adhesiveness for the vascular endothelium and other blood cells. Numerous protein-protein interactions between the malaria-parasite and the host RBC are important for many aspects of parasite biology and the pathogenesis of malaria. In addition, there are many other parasite proteins located within the infected red cell and at the membrane skeleton, for which no precise functional roles have yet been elucidated. Sequencing and annotation of the complete genome of Plasmodium falciparum, the production of proteomic and transcriptomic profiles of parasites, and the development of a transfection system for the asexual stage of the parasite are all recent achievements that should advance understanding of the molecular mechanisms that underlie the parasite-induced functional alterations in red cells.

Immunization with PfEMP1-DBL1alpha generates antibodies that disrupt rosettes and protect against the sequestration of Plasmodium falciparum-infected erythrocytes

A family of parasite antigens known as Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) is believed to play an important role in the binding of infected erythrocytes to host receptors in the micro-vasculature. Available data advocates the existence of a subset of very adhesive (rosetting, auto-agglutinating) and antigenic PfEMP1s implicated as virulence factors. Serum antibodies that disrupt rosettes are rarely found in children with severe malaria but are frequent in those with mild disease suggesting that they may be protective. Here we have developed a Semliki forest virus (SFV) vaccine construct with a recombinant gene (mini-var gene) encoding a mini-PfEMP1 (DBL1alpha-TM-ATS) obtained from a particularly antigenic and rosetting parasite (FCR3S1.2). The mini-PfEMP1 is presented to the host mimicking the location of the native molecule at the infected erythrocyte surface. Antibodies generated by a regimen of priming with SFV RNA particles and boosting with a recombinant protein recognize the infected erythrocyte surface (immuno-fluorescence/rosette-disruption) and prevent the sequestration of P. falciparum-infected erythrocytes in an in vivo model of severe malaria. The data prove the involvement of DBL1alpha in the adhesion of infected- and uninfected erythrocytes and the role of rosette-disruptive antibodies in preventing these cellular interactions. The work supports the use of DBL1alpha in a vaccine again severe malaria.

Plasmodium falciparum associated with severe childhood malaria preferentially expresses PfEMP1 encoded by group A var genes

Parasite-encoded variant surface antigens (VSAs) like the var gene-encoded Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) family are responsible for antigenic variation and infected red blood cell (RBC) cytoadhesion in P. falciparum malaria. Parasites causing severe malaria in nonimmune patients tend to express a restricted subset of VSA (VSA(SM)) that differs from VSA associated with uncomplicated malaria and asymptomatic infection (VSA(UM)). We compared var gene transcription in unselected P. falciparum clone 3D7 expressing VSA(UM) to in vitro-selected sublines expressing VSA(SM) to identify PfEMP1 responsible for the VSA(SM) phenotype. Expression of VSA(SM) was accompanied by up-regulation of Group A var genes. The most prominently up-regulated Group A gene (PFD1235w/MAL7P1.1) was translated into a protein expressed on the infected RBC surface. The proteins encoded by Group A var genes, such as PFD1235w/MAL7P1.1, appear to be involved in the pathogenesis of severe disease and are thus attractive candidates for a vaccine against life-threatening P. falciparum malaria.

Malaria immunity in infants: a special case of a general phenomenon?

Newborn infants in endemic areas are markedly resistant to Plasmodium falciparum malaria. Consequently, severe disease is rare during the first few months of life, and infections tend to be low density and relatively asymptomatic during this period. Although this is generally ascribed to passively transferred immunity, attempts to identify the targets and mechanisms of this protection have been unsuccessful. The implications of the hypothesis that the progression from resistance through susceptibility and back to resistance during infancy and early childhood reflects the gradual acquisition of IgG to variant surface antigens (VSAs), while protection from maternal VSA-specific IgG steadily fades, are discussed here.

Geographical and temporal conservation of antibody recognition of Plasmodium falciparum variant surface antigens

The slow acquisition of protection against Plasmodium falciparum malaria probably reflects the extensive diversity of important antigens. The variant surface antigens (VSA) that mediate parasite adhesion to a range of host molecules are regarded as important targets of acquired protective immunity, but their diversity makes them questionable vaccine candidates. We determined levels of VSA-specific immunoglobulin G (IgG) in human plasma collected at four geographically distant and epidemiologically distinct localities with specificity for VSA expressed by P. falciparum isolates from three African countries. Plasma levels of VSA-specific IgG recognizing individual parasite isolates depended on the transmission intensity at the site of plasma collection but were largely independent of the geographical origin of the parasites. The total repertoire of immunologically distinct VSA thus appears to be finite and geographically conserved, most likely due to functional constraints. Furthermore, plasma samples frequently had high IgG reactivity to VSA expressed by parasites isolated more than 10 years later, showing that the repertoire is also temporally stable. Parasites from patients with severe malaria expressed VSA (VSASM) that were better recognized by plasma IgG than VSA expressed by other parasites, but importantly, VSASM-type antigens also appeared to show substantial antigenic homogeneity. Our finding that the repertoire of immunologically distinct VSA in general, and in particular that of VSASM, is geographically and temporally conserved raises hopes for the feasibility of developing VSA-based vaccines specifically designed to accelerate naturally acquired immunity, thereby enhancing protection against severe and life-threatening P. falciparum malaria.

Identification of a 67-amino-acid region of the Plasmodium falciparum variant surface antigen that binds chondroitin sulphate A and elicits antibodies reactive with the surface of placental isolates

The complications of malaria in pregnancy are caused by the massive sequestration of parasitized erythrocytes (PE) in the placenta. Placental isolates of Plasmodium falciparum are unusual in that they do not bind the primary microvasculature receptor CD36 but instead bind chondroitin sulphate A (CSA). Pregnant mothers develop antibodies that recognize placental variants worldwide, suggesting that a vaccine against malaria in pregnancy is possible. Some members of the Duffy binding-like gamma (DBL-gamma) domain of the large and diverse P. falciparum erythrocyte membrane protein-1 (PfEMP-1) family, when expressed on Chinese hamster ovary (CHO) cells, bind CSA. To characterize better the molecular requirements for DBL-gamma adhesion to CSA, we determined the binding of various DBL-gamma domains. Most DBL-gamma did not bind CSA, and no conserved region was identified that strictly differentiated binders from non-binders. Structure-function analysis of the FCR3-CSA DBL-gamma domain localized the minimal CSA binding region to a 67-residue fragment. This region was partially conserved among some binding sequences. Serum from a rabbit immunized with the minimal domain reacted with CSA-binding parasite lines, but not with non-CSA-adherent PE lines that adhered to CD36 and other receptors. The identification of a minimal binding region from a highly variable cytoadherent family may have application for a vaccine against malaria in pregnancy.

Sequestration: causes and consequences

Adhesion of Plasmodium falciparum-infected erythrocytes to endothelial cells and to syncytiotrophoblasts lining the placenta is a key feature of malaria pathogenesis. P. falciparum erythrocyte membrane protein 1, a family of variable proteins, mediates adhesion to CD36 and intercellular adhesion molecule 1 in the systemic vasculature, and to chondroitin sulphate A and hyaluronic acid in the placenta. Recent studies of the pathology of fatal cerebral malaria and of placental malaria that follow such sequestration suggest that coagulation disturbances may have a greater role in pathogenesis than previously realized, and that monocyte infiltrates in response to malaria may initiate some of these changes. Chemokines such as macrophage inflammatory protein 1 alpha and beta and monocyte chemoattractant protein 1 may play a key role in attracting monocytes to the placenta and other organs, but the stimulus to chemokine secretion is not presently known.

Detecting selection using a single genome sequence of M. tuberculosis and P. falciparum

Selective pressures on proteins are usually measured by comparing nucleotide sequences. Here we introduce a method to detect selection on the basis of a single genome sequence. We catalogue the relative strength of selection on each gene in the entire genomes of Mycobacterium tuberculosis and Plasmodium falciparum. Our analysis confirms that most antigens are under strong selection for amino-acid substitutions, particularly the PE/PPE family of putative surface proteins in M. tuberculosis and the EMP1 family of cytoadhering surface proteins in P. falciparum. We also identify many uncharacterized proteins that are under strong selection in each pathogen. We provide a genome-wide analysis of natural selection acting on different stages of an organism's life cycle: genes expressed in the ring stage of P. falciparum are under stronger positive selection than those expressed in other stages of the parasite's life cycle. Our method of estimating selective pressures requires far fewer data than comparative sequence analysis, and it measures selection across an entire genome; the method can readily be applied to a large range of sequenced organisms.

Molecular aspects of malaria pathogenesis

Plasmodium falciparum being the most lethal plasmodiae is still a major cause of the disease burden and mortality in malaria endemic areas. Due to the wide spread drug resistance in combination with poor socio-economic situation in the vast majority of the endemic countries, malaria is today a great global challenge. The scientific community is, however, progressing. The 23 Mb genome of P. falciparum has been decoded and publicly available. Data of transcriptional profiling at certain developmental stages have already been generated. More than 50% of P. falciparum genes are transcribed constitutively in all the developmental stages of parasite life cycle. Functional disruption of these genes might have implications for parasite growth and development. Available microarray data indicate that P. falciparum preferentially expresses rif and stevor gene families at gametocyte and sporozoite stages while var genes are predominantly expressed at the erythrocytic stage. Gene regulation mechanisms of the variant gene families in P. falciparum are still not understood though some regulatory elements have been proposed. The occurrence of severe malaria is determined by both parasite and human host factors. Sequestration and antigenic variation are two of the evasion mechanisms utilized by P. falciparum in order to escape the human host defences. Understanding the molecular mechanisms underlying these phenomena is of a major importance and interest in malaria research. Here, we summarize and highlight the recent progress in molecular aspects of severe malaria.

Evidence for the importance of genetic structuring to the structural and functional specialization of the Plasmodium falciparum var gene family

The var gene family encodes Plasmodium falciparum erythrocyte membrane 1 (PfEMP1) proteins that act as virulence factors responsible for both antigenic variation and cytoadherence of infected erythrocytes. These proteins orchestrate infected erythrocyte sequestration from blood circulation and contribute to adhesion-based complications of P. falciparum malaria infections. For this study, we analysed the genetic organization and strain structure of var genes and present evidence for three separately evolving groups that have, in part, functionally diverged and differ between subtelomeric and central chromosomal locations. Our analyses suggest that a recombination hierarchy limits reassortment between groups and may explain why some var genes are unusually conserved between parasite strains. This recombination hierarchy, coupled with binding and immune selection, shapes the variant antigen repertoire and has structural, functional and evolutionary consequences for the PfEMP1 protein family that are directly relevant to malaria pathogenesis.

var genes, PfEMP1 and the human host

Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) is an important virulence factor encoded by a family of roughly 60 var genes and is used by the parasite to interact with the human host. The parasite regularly exchanges the expressed var gene generating antigenic variation of the infected RBCs (pRBC) surface which is crucial for successful proliferation and transmission. PfEMP1 is also an adhesive molecule that binds to an array of human receptors. By sequestration in the post-capillary venules, pRBCs are able to escape the spleen-mediated clearance but severe malaria may develop if the local binding is extensive. Anti-PfEMP1 immunity is important for preventing the development of both cerebral malaria and placental malaria, but more immunological studies on PfEMP1 antigens and their interaction with the human host are needed. Over the last few years our knowledge about var genes and PfEMP1s has increased dramatically through genetic, biochemical, immunological and epidemiological studies. In addition, the genome sequence has also provided us with a new platform for further dissecting its biological functions. This review highlights the recent analyses of var genes in the P. falciparum genome and postulates significance of genome recombination to the diversity of parasite virulence.